US6237368B1 - Process for the regulating or controlling the NOx content of exhaust gases given off during the operating of glass melting furnaces with several burners run alternately - Google Patents

Abstract

A method for regulating or controlling the content of NO_x in the exhaust gases of a glass-melting furnace having several burners operated in alternation, wherein both the beginning and the end of a combustion break (FP⁺, FP⁻) are supplied to a binary signal generator (8) which passes a signal to a regulator (4) with a time delay and upon receipt of the time-regulator (4), and the amount of NH₃ supplied to the denitrating plant is adjusted to a lower constant fixed value F1 via a control, and by means of a memory element (7) a higher constant fixed value F2 is calculated as amount of NH₃ and supplied to the regulator (4), whereupon as soon as the regulator (4) has received the signal of the time-delayed end of a combustion break, the fixed value F1 is adjusted to the fixed value F2 via a control, and subsequently the regulation is directly continued.

Description

This invention relates to a method of regulating or controlling the content of NO_x in exhaust gases produced during the operation of glass-melting furnaces with several burners which are operated in alternation.

BACKGROUND OF THE INVENTION

Methods of reducing nitrogen oxides in exhaust gases are known. The DE-OS-3615021 describes a method for the selective catalytic reduction of nitrogen oxides from exhaust gases of internal combustion engines by adding ammonia in a reactor. In accordance with this method the addition of ammonia is effected in dependence on the NO_x-concentration in the exhaust gas, and the NO_x-concentration is determined indirectly by measuring operating parameters of the internal combustion engine and subsequently calculating the concentration of nitrogen monoxide and nitrogen dioxide in dependence on at least one selected operating parameter on the internal combustion engine in consideration of families of characteristics.

In Römpps Chemie-Lexikon, 8th edition, pp. 1484 to 1490 the operation of glass-melting furnaces is described in detail. Glass-melting furnaces mostly are tank furnaces having a plurality of laterally disposed burners which are operated in alternation. The actual heating of the glass-melting furnaces is mostly effected by means of long-distance gas, heating oil or natural gas. The exhaust gases produced contain nitrogen oxides, due to fuels, high temperatures or additives. During the denitrification of exhaust gases, the NO_x-content of the pure gas must, for legal reasons, always be monitored in connection with the O₂-content of the pure gas, which leads to the fact that in practice the setpoint of the NO_x-content, NO_xset, is transformed into a standardized setpoint NO_xset n. In general, the following relation is used for the standardization: ${\mathrm{NO}}_x\mathrm{set}n={\mathrm{NO}}_x\mathrm{set}·\frac{\left(21-O_2\mathrm{act}\right)}{\left(21-8\right)}$

However, this standardization is disadvantageous when the glass-melting furnaces comprise several burners which are operated in alternation. If one burner is switched off during a combustion break, the NO_x-content of the exhaust gas drops to a relatively large extent. When regulating the content of NO_x in the exhaust gases by means of a simple regulator circuit, the introduced amount of NH₃, which reacts with the nitrogen oxides in a known manner, is dependent on the deviation xd, wherein:

xd=NO_xsetn−NO_x′

With decreasing NO_x-content of the exhaust gases both the value NO_xset n and the value NO_x′ are decreased, which leads to the fact that the deviation xd does not or only insignificantly change. Since with a reduction of the content of NO_x in the exhaust gases the deviation xd changes only insignificantly, the amount of NH₃ to be supplied likewise remains almost constant in the denitrating plant, which leads to the fact that more NH₃ is introduced than can be reacted with the nitrogen oxides. This in turn leads to the fact that the content of NH₃ in the pure gas generally exceeds the admissible limit values. A further disadvantage of this conventional known regulation lies in the fact that the denitrating plant is generally not arranged in direct vicinity of the glass-melting furnaces. Thus, the exhaust gas requires some time to flow from the glass-melting furnace to the pure-gas port of the denitrating plant, in which port the pure gas values are measured in general. When the operation of a burner is interrupted, a NO_x-content is measured in the denitrating plant which requires a higher amount of NH₃ than this is actually necessary with the real values in the glass-melting furnace. Thus, a certain time must elapse before a regulation by means of a simple regulator circuit can be performed to react on the individual combustion breaks of the burners in the glass-melting furnaces.

SUMMARY OF THE INVENTION

The object underlying the invention is to provide a method of regulating or controlling the content of NO_x in exhaust gases produced during the operation of glass-melting furnaces with several burners which are operated in alternation, where the known standardization of the setpoint NO_xset need not be omitted. By means of this method a relatively quick reaction to fluctuating NO_x-contents during combustion breaks of individual burners in the glass-melting furnace should furthermore be possible.

DETAILED DESCRIPTION

The object underlying the invention is solved by a method of regulating or controlling the content of NO_x in exhaust gases produced during the operation of glass-melting furnaces with several burners which are operated in alternation, where the setpoint of the NO_x-content, NO_xset, is supplied to a multiplier, at the same time the content of O₂ in the pure gas, O₂act, is measured continuously, and the content of O₂ detected in a first transducer, O₂act′, is likewise supplied to the multiplier, and in the multiplier a standardization of the setpoint NO_xset into a standardized setpoint NO_xset n is effected, where the following applies for the standardization: ${\mathrm{NO}}_x\mathrm{set}n={\mathrm{NO}}_x\mathrm{set}·\frac{\left(21-O_2\mathrm{act}\right)}{\left(21-8\right)}$

and where the standardized setpoint NO_xset n is compared with the content of NO_x in the pure gas, NO_x′, which has been detected by a second transducer, the deviation xd resulting from such comparison is supplied to a regulator, which adapts the amount of NH₃ to be supplied to the denitrating plant as a correcting variable y for regulating the NO_x-content as a regulating variable, and where both the beginning of a combustion break FP⁺ and the end of a combustion break FP⁻ are each supplied as a signal to a binary signal generator, which supplies the signals with a time delay as time-delayed beginning of a combustion break FP_z ⁺ or as time-delayed end of a combustion break FP_z ⁻ to the regulator, which interrupts the regulation upon receipt of the signal FP_z ⁺ and adjusts the amount of NH₃ to a lower constant fixed value F1 by means of a control, the content of NO_x in the pure gas, NO_x′, detected by the second transducer is supplied to a memory element, where it is transformed into a higher constant fixed value F2 as amount of NH₃, which is likewise supplied to the regulator, and where, as soon as the regulator has received the signal FP_z ⁻, the fixed value F1 is adjusted to the fixed value F2 via a control, and directly subsequent thereto the regulation is continued. As glass-melting furnaces there are generally used pot furnaces or tank furnaces, which operate continuously or discontinuously and comprise several burners. The term“burner” not only includes the heatings with long-distance gas, heating oil or natural gas, but also heating electrodes. The term “combustion break” refers to the interruption of the operation of at least one burner. The signal for the beginning of a combustion break FP⁺ is immediately generated whenever the burner is switched off. The signal for the end of a combustion break FP⁻ is immediately generated whenever the burner is switched on again. The signal for the time-delayed beginning of a combustion break FP_z ⁺ is generated by the binary signal generator a certain period after the burner has been switched off. The signal for the time-delayed end of a combustion break FP_z ⁻ is generated by the binary signal generator a certain period after the burner has been switched on again. In the definition of this time delay Δt, which in both cases is the same, two definitions are required for technical reasons. When the temperature of the exhaust gases lies between 750 and 1100° C., the denitrification can be effected by addition of NH₃ without a catalyst being present. In this case, Δt is the time required by the exhaust gas to flow from the glass-melting furnace to the point where NH₃ is introduced into the denitrating plant. When the temperatures of the exhaust gases lie in the range between 300 and 450° C., the denitrification not only requires a contacting with NH₃, but also a contacting with an appropriate catalyst, for instance titanium dioxide. In this case Δt is the time required by the exhaust gas to flow from the glass-melting furnace to that point in the denitrating plant, where it is for the first time both contacted with NH₃ and with the used catalyst. The lower constant fixed value F1 represents 5 to 20% of the amount of NH₃ introduced directly before the interruption of the regulation. During the transformation of the fixed value F2, values of the content of NO_x in the pure gas, NO_x′, are used as starting values, where the memory element can operate in different ways. The fixed value F2 can for instance represent that amount of NH₃ which was necessary for adjusting the last-measured content of NO_x in the pure gas. From the last-measured contents of NO_x in the pure gas average values can, however, be formed advantageously, from which then the fixed value F2 can be calculated. It has surprisingly turned out that by means of the inventive method the disadvantages of the known standardization can be eliminated, where it is possible at the same time to relatively quickly react to fluctuating NO_x-contents in the glass-melting furnaces, due to the combustion breaks of the individual burners. In the method in accordance with the invention, the admissible limit values of NH₃ in the pure gas are thus not exceeded.

In accordance with a preferred aspect of the invention the constant fixed value F1 is 6 to 15% of the amount of NH₃ introduced directly before the interruption of the regulation. This is generally not enough for sufficiently converting the still existing content of NO_x in the exhaust gases, where at the same time it can advantageously and easily be avoided that the admissible limit values of NH₃ in the pure gas are exceeded.

In accordance with a further preferred aspect of the invention the transformation is effected in a memory element through formation of an average, formed from the contents of NO_x in the pure gas, NO_x′, which were measured over a period of 5 to 40 min. Advantageously, the regulation can be continued with a fixed value F2, which is relatively close to the optimum amount of NH₃ to be supplied, when the signal FP_z ⁻ is generated by the binary signal generator, i.e. at the time tFP_z ⁻.

In accordance with a further aspect of the invention the period is 12 to 18 min. In general, this period is sufficient to mostly obtain a transformed fixed value F2, by means of which the regulation can be continued quickly and easily. To a particular advantage, the period is 15 min.

In accordance with a further preferred aspect of the invention, the exhaust gases are liberated from SO_x, HCl, HF and dust prior to the removal of NO_x upon leaving the glass-melting furnaces. This has a particularly advantageous effect on the execution of the method in accordance with the invention, as disadvantageous influences, due to the noxious substances SO_x, HCl, HF and dust, are eliminated. The removal of SO_x, HCl and HF at 300 to 500° C. can advantageously be effected in a classical or circulating fluidized bed or in an entrained-bed reactor. For removing SO_x, HCl and HF, the exhaust gases are for instance contacted with Ca(OH)₂.

In accordance with a further aspect of the invention, the exhaust gases are first of all liberated from SO_x, HCl and HF and then passed through an electrostatic dust separator. The electrostatic dust separators used are electrostatic filters operating dry. Advantageously, the electrostatic separator is not contacted with SO_x, HCl and HF, which provides for a relatively small maintenance effort.

In accordance with a further aspect of the invention, the removal of SO_x, HCl and HF is effected in a fluidized bed through addition of Ca(OH)₂. This provides for a relatively complete removal of the noxious substances SO_x, HCl and HF with a high efficiency.

The invention will now be explained in detail and by way of example with reference to the drawing (FIGS. 1 to 3).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the signal flow diagram in accordance with DIN 19226 of the inventive process for regulating or controlling the NO_x-content.

FIG. 2 shows the signal flow diagram in accordance with DIN 19226 of the known regulation of the content of NO_x in exhaust gases produced during the operation of glass-melting furnaces with several burners which are operated in alternation.

FIG. 3 shows by way of example the flow rate of the amount of NH₃, {dot over (V)}_{NH 3} , to be supplied to the denitrating plant, as a function of the time t in accordance with the inventive method.

FIG. 1 represents the inventive method of regulating or controlling the content of NO_x in exhaust gases. The setpoint NO_xset predetermined by the setpoint transmitter 1 is supplied to the multiplier 2. At the same time, the content of O₂ in the pure gas, O₂act, is measured, and the content of O₂ detected in the first transducer 3, O₂act′, is likewise supplied to the multiplier 2. In the multiplier 2 the setpoint NO_xset is standardized to obtain the standardized setpoint NO_xset n in consideration of the content O₂act′ detected in the first transducer 3. The standardized setpoint NO_xset n is compared with the content of NO_x in the pure gas, NO_x′, which was detected in the second transducer 6. The resulting deviation xd is supplied to a regulator 4, which adapts the amount of NH₃ to be supplied to the denitrating plant 5 as a correcting variable y for regulating the NO_x-content as a regulating variable. The regulator 4 is understood to be the combination of regulator, actuator and controller. The signal of the beginning of a combustion break FP⁺ and the signal of the end of a combustion break FP⁻ is each supplied to a binary signal generator 8. With a time delay, the binary signal generator 8 supplies the signals FP_z ⁺ or FP_z ⁻ to the regulator 4. Upon receipt of the signal FP_z ⁺ the regulation is interrupted by the regulator 4, and the amount of NH₃ is adjusted to a lower constant fixed value F1 via a control. The content of NO_x in the pure gas, NO_x′, which was detected by the second transducer 6, is supplied to a memory element 7, where it is transformed to a higher constant fixed value F2 as amount of NH₃. The memory element 7 is advantageously used for the formation of the average, formed from the content of NO_x in the pure gas, NO_x′, measured over a period of 5 to 30 min, from which then the associated fixed value F2 is calculated as amount of NH₃ and supplied to the regulator 4. As soon as the regulator 4 has received the signal FP_z ⁻, the fixed value F1 is switched over to the fixed value F2 via a control. Directly thereafter, the regulation is continued. Prior to the removal of NO_x, the exhaust gases can advantageously be liberated from SO_x, HCl, HF and dust (not represented), as soon as they have left the glass-melting furnaces.

FIG. 2 represents the generally known regulation of the content of NO_x in exhaust gases produced during the operation of glass-melting furnaces with several burners which are operated in alternation, by means of a simple regulator circuit. The known standardization and the slow regulation due to the relatively long distance to be covered by the exhaust gases from the point of introduction of NH₃ to the point where the pure gas is measured in the denitrating plant 5 have a disadvantageous effect in this known regulation.

FIG. 3 represents by way of example the function of the amount of NH₃ to be supplied to the denitrating plant, {dot over (V)}_{NH 3} , as a function of the time t. 15 min before the time at which the signal FP⁺ is generated, i.e. 15 min before tFP⁺, the individual measured contents of NO_x in the pure gas, NO_x′, are stored in the memory element 7, the average is formed, and subsequently a fixed value F2 is defined as amount of NH₃. After the time delay Δt, at the point tFP_z ⁺, at which the signal FP_z ⁺ is passed on from the binary signal generator 8 to the regulator 4, the regulation is interrupted and the graph of the function abruptly drops to a fixed value F1 and is kept constant. When the signal of the end of a combustion break FP⁻ is generated at the point tFP⁻, the graph will only rise again from the fixed value F1 to the fixed value F2 upon expiration of the time delay Δt precisely at the point tFP_z ⁻, at which the signal FP_z ⁻ is supplied from the binary signal generator to the regulator 4. The advantage is that there is a relatively fast change-over from the fixed value F1 to the fixed value F2 at the point tFP_z ⁻, and this change-over is not connected with any regulation-related delay. Directly at the point tFP_z ⁻ the fixed value F2 is reached, and proceeding from this fixed value F2, which is very close to the optimum value of the amount of NH₃ to be supplied at this time, the regulation may be continued in an advantageous manner. The time delay Δt either is the time required by the exhaust gas to flow from the glass-melting furnace to the point of introduction of NH₃ or the time required by the exhaust gas to flow from the glass-melting furnace to that point where the exhaust gas is contacted for the first time with NH₃ and a catalyst, for instance titanium dioxide. This depends on the temperatures of the exhaust gas. In FIG. 3 the 15-minute period is illustrated only by way of example. It may comprise a period of 5 to 40 min, advantageously 12 to 18 min.

Claims (7)

What is claimed is:

1. A method of regulating or controlling the content of NO_x in exhaust gases released from a denitrating plant associated with the operation of glass-melting furnaces with several burners which are operated in alternation, wherein the setpoint of the content of NO_x, NO_xset, is supplied to a multiplier (2), at the same time the content of O₂ in the released gas, O₂act, is measured continuously, and the content of O₂ detected in a first transducer (3), O₂act′, is likewise supplied to the multiplier (2), and in the multiplier (2) a standardization of the setpoint NO_xset into a standardized setpoint NO_xset n is effected, where the following applies for the standardization: ${\mathrm{NO}}_x\mathrm{set}n={\mathrm{NO}}_x\mathrm{set}·\frac{\left(21-O_2\mathrm{act}\right)}{\left(21-8\right)}$

and where the standardized setpoint NO_xset n is compared with the content of NO_x in the released gas detected by a second transducer (6), NO_x′, the deviation xd resulting from this comparison is supplied to a regulator (4), which adapts an amount of NH₃ to be supplied to the denitrating plant (5) as a correcting variable y for reacting with and regulating the content of NO_x as regulating variable, and where both the beginning of a combustion break FP⁺ and the end of a combustion break FP⁻ are each supplied as a signal to a binary signal generator (8), which with a time delay supplies the signals as time-delayed beginning of a combustion break FP_z ⁺ or as time-delayed end of a combustion break FP_z ⁻ to the regulator (4), which interrupts the regulation upon receipt of the signal FP_z ⁺ and adjusts the amount of NH₃ to a lower constant fixed value F1 via a control, the amount of NO_x in the released gas detected by the second transducer (6), NO_x′, is supplied to a memory element (7), transformed there into a higher constant fixed value F2 as amount of NH₃, and is likewise supplied to the regulator (4), and where, as soon as the regulator (4) has received the signal FP_z ⁻, the fixed value F1 is adjusted to the fixed value F2 via a control, whereafter the regulation is continued immediately.

2. The method according to claim 1, wherein the constant fixed value F1 is 6 to 15% of the amount of NH₃ introduced immediately before the interruption of the regulation.

3. The method according to claim 1, wherein where the transformation in the memory element (7) is effected through formation of an average, formed from the contents of NO_x in the released gas, NO_x′, measured over a period of 5 to 40 min.

4. The method according to claim 3, wherein the period comprises 12 to 18 min.

5. The method according to claim 1, wherein the exhaust gases are liberated from SO_x, HCl, HF and dust prior to the removal of NO_x upon leaving the glass-melting furnaces.

6. The method according to claim 5, wherein the exhaust gases are first liberated from SO_x, HCl and HF and are then passed through an electrostatic dust separator.

7. The method according to claim 6, wherein the removal of SO_x, HCl and HF is effected in a fluidized bed through addition of Ca(OH)₂.

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